The present invention relates to an estimation coding system for efficiently transmitting or storing an image of variable density. More specifically, the invention relates to an adaptive type quantizer which controls quantization characteristics according to variations of input picture elements.
A variety of coding compression systems have been proposed in the art which are intended to reduce the transmission time and the transmission cost in transmitting an image of variable density in a narrow bandwidth. One of the conventional systems is a DPCM (Differential Pulse Code Modulation) system. In accordance with that system, the picture element value for a present point is estimated from past picture element values and the estimation error, which is the difference between the estimation value and the picture element value of the present point, is quantized and coded thus achieving the desired bandwidth compression. In general, in such an estimation coding system, in order to improve the compression effect, nonlinear quantization is employed to make the quantization step intervals small when the estimation error is small and larger when the estimation error is large taking into account the visual characteristic that visual sensitivity is low for an image area where the density changes abruptly but is sufficiently high for an image area of fairly uniform density and the number of quantization steps is made as small as possible.
However, if the number of quantization steps is made excessively small, the quantization error is increased as a result of which the system cannot sufficiently respond to abrupt variations in density of an input image thereby causing gradient overload noise in the reproduced image. Accordingly, in order to prevent the above-described degradation of an image between areas of variable density areas of fairly uniform density, an adaptive type quantization system in which the quantization characteristic is varied adaptively according to the variations of input image values has been proposed in which a quantization characteristic having a large quantization level value is used when the input image value varies greatly and a quantization characteristic having a small quantization level value is used when the input image value varies less.
In one of the conventional systems of this type, the quantization characteristic is determined with the present quantization level value being used to estimate the quantization level value for the next picture element. Accordingly, the quantization characteristic varies for almost all the input picture elements with the result that the reproduced image is greatly degraded.
This difficulty has been eliminated by another conventional system in which quantization level values corresponding to input picture elements are monitored for a certain period of time and the quantization characteristic is changed when required. However, the conventional system is also disadvantageous in that, as described above, while the quantization level values variable with time are being monitored, a change of the quantization characteristic is watched for. Accordingly, when the quantization level value changes extremely frequently, the quantization characteristic is also frequently changed. In this case, the probability of selecting a quantization characteristic of large quantization step interval for input picture elements to be quantized is considerably high although a quantization characteristic of a small quantization step interval should be selected therefor so that the reproduced image is significantly degraded.
Furthermore, these conventional systems are disadvantageous in that propagation therein is greatly affected by the transmission error according to the degree of variation of the quantization characteristic.